1. Field of the Invention
The present invention relates to an electrode wire for wire electrical discharge machining that machines a workpiece (an object being machined) with electrical discharges and a method for manufacturing the electrode wire and a method for electrical discharge machining with the electrode wire.
2. Description of the Related Art
The wire electrical discharge machining is used to cut a workpiece by means of heat energy produced by electrical discharges created between an electrode wire for electrical discharge machining and the workpiece, and is particularly suitable for metalworking for producing metal molds or other intricate-shaped metal items.
Electrical discharge machining needs to meet certain requirements, for example, a) high machining speed; b) excellent surface finish and accurate dimension of a workpiece; c) high positioning accuracy for determining the position of the electrode wire relative to the workpiece; and d) low metal debris production caused by the continuously running electrode wire.
A widely-used conventional electrode wire is a solid brass electrode wire with a zinc concentration of 35 to 40 weight %. If the content of zinc is increased to 40 weight % or higher, the solid brass electrode wire produces an intermetallic compound with a body-centered cubic lattice that deteriorates malleability, ductility and toughness of the wire. Such a solid brass electrode wire cannot be subjected to cold drawing and therefore is impossible to be manufactured.
Various research on solid brass electrode wires for making the electrical discharge machining speed higher than the solid brass electrode wire with a zinc concentration of 35 to 40 weight % frequently suggests that the electrical discharge machining speed can be accelerated with an increase in zinc concentration in the composition of the electrode wire.
As a method for increasing the zinc concentration, it is known to provide a copper-zinc alloy layer with a zinc concentration of 40 weight % or higher on the surface of an electrode wire, and optionally add a zinc layer on the copper-zinc alloy layer.
Japanese Patent No. 3718617 (Patent Document 1) discloses a porous electrode wire provided on its surface with a copper-zinc alloy layer with a zinc concentration of at least 40 weight % and optionally provided with a zinc layer on the copper-zinc alloy layer.
The porous electrode wire is made of a copper bearing core, a copper-zinc alloy layer on the surface of the core, and optionally a zinc layer on the copper-zinc alloy layer, those layers being provided through hot-dip galvanizing, and is drawn to intentionally crack its surface to increase the surface area of the electrode wire. The increased surface area increases the contact area between the wire and machining liquid during electrical discharging, thereby further accelerating the cooling speed and accordingly enhancing the machining speed.
In addition, International Publication No. WO 2009/028117 (Patent Document 2) discloses an electrode wire having the following configurations for the purpose of addressing the problem of the above invention.                The electrode wire includes an inner copper-zinc alloy layer (a zinc concentration of 50 to 80 weight %) formed by thermal diffusion of molten zinc into a core made of copper or copper alloy and an outer copper-zinc alloy layer (a zinc concentration of 81 to 100 weight %) formed by diffusion of copper of the core into the molten zinc (three-layer structure including the two copper-zinc alloy layers, the outer layer of which being formed by diffusion, and a zinc layer provided thereon).        The zinc layer is thicker than the diffusion alloy layer.        The thickness of the zinc layer constitutes at least 1.2% of the outer diameter, and no cracks are present on the outermost layer of the electrode wire.        
It is possible to improve the machining speed with the conventional high-speed machining electrode wires formed by providing a zinc layer and a diffusion alloy layer on the circumference surface of a core made of copper or copper alloy and drawing it, but the wires deteriorate the other properties required for electrical discharge machining.
Forming cracks in the surface layer of an electrode wire is a well-known technique; however, the cracks formed in the surface layer of the electrode wire cause the following problems:
a. Since wire electrical discharge machining is to cut a workpiece, like a jig saw, by producing discharges between an electrode wire and workpiece, cracks in the surface layer of the electrode wire destabilize the electrical discharges, resulting in poor surface finish of the workpiece;
b. Wire electrical discharge machining apparatuses recognize the relative position between the workpiece and electrode wire by utilizing the electrical conductivity between the workpiece and electrode wire. The presence of the cracks in the surface layer of the electrode wire reduces the contact area and therefore deteriorates the positioning accuracy;
c. Such a brittle surface as is cracked when cold drawn produces a large amount of metal debris due to friction and chafing between the electrode wire and a guide, pulley and some other parts of a machining apparatus that continuously runs the electrode wire for electric discharge machining; and
d. The cracks in the surface layer decrease reliability of the electrode wire because the wire is likely to break during handling or machining.
Well-known measures are taken to prevent cracks from generating in the outermost layer of an electrode wire with a diffusion alloy layer. This electrode wire is designed to include a zinc layer having a thickness constituting 1.2% or more of the outer diameter of the wire to cover the cracks in the diffusion alloy layer, and therefore the outermost layer, which is the zinc layer, needs to be thick to provide the outermost layer without cracks. However, such a thick zinc layer wears out due to evaporation of zinc and becomes small in diameter during electric discharge machining as shown in FIG. 4, thereby making a difference in width of a machined groove between an inlet side and outlet side and consequently tapering the surface being machined, which is a problem in machining accuracy.
In addition, the thick zinc layer and diffusion alloy layer are likely to peel off from the core after being subjected to a wire drawing process to obtain an electrode wire having a desired diameter of approximately 0.1 to 0.3 mmφ.
If the electrode wire with such an easy-to-peel diffusion alloy layer and zinc layer is used to perform electrical discharge machining, peeled pieces form a bridge between the electrode wire and workpiece and the bridge brings about shorts. Deceleration of machining-speed due to reduction of the number of electrical charges and unstable electrical discharges produce dense collections of craters appearing as fine streaks on the machined surface along the travelling direction of the electrode wire as shown in a schematic diagram of FIG. 5.